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#118: Add an attribute in Appendix F to identify the geoid
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Reporter: jonathan | Owner: cf-conventions@…
Type: enhancement | Status: new
Priority: medium | Milestone:
Component: cf-conventions | Version:
Resolution: | Keywords:
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Comment (by jonathan):
Dear Mark
I don't think we should reopen the extremely lengthy and effortful
discussions which we had before about how to approach the relationship
with WKT. In the end, we agreed two tickets. Ticket 69 allows WKT to be
included in the `grid_mapping` variable. It says, "The `crs_wkt` attribute
is intended to act as a supplement to other single-property CF grid
mapping attributes (as described in Appendix F); it is not intended to
replace those attributes. If data producers omit the single-property grid
mapping attributes in favour of the compound `crs_wkt attribute`, software
which cannot interpret `crs_wkt` will be unable to use the `grid_mapping`
information. Therefore the CRS should be described as thoroughly as
possible with the single-property attributes as well as by `crs_wkt`."
Ticket 80 adds some more attributes to `grid_mapping` so that more of the
WKT can be put into CF attributes. Etienne (or perhaps it was Phil
Bentley, I'm sorry I'm not sure which) extracted lists from the EPSG
database for convenience of users. Those extracted lists themselves are
not definitive; we're not maintaining a CF standard, but relying on an
external standard.
The present ticket is a supplement to ticket 80, in effect. CF doesn't say
anything about the legal syntax of WKT strings, because that's not our
business. However, we do need a CF method to specify the geoid, for the
sake of precision in defining coordinates which refer to the geoid.
If there is a `grid_mapping` attribute of the kind you can have in CF-1.6
and earlier versions, it applies to all the coordinates of the data
variable. If you did want to use different geoids for different
coordinates, that could be done with extended form of `grid_mapping`
attribute that we agreed on a ticket of yours.
Cheers
Jonathan
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Ticket URL: <http://kitt.llnl.gov/trac/ticket/118#comment:5>
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